The interferometer is known as a very accurate optical measurement tool for measuring a physical quantity by means of the measurand-induced changes of the interferometer path length difference. When using a narrowband light source, the coherence length of the source is generally greater than the path length difference of the interferometer and therefore the measurement suffers from a 2π phase ambiguity, due to the periodic nature of the interferogram fringes, which may severally restricts the measuring applications. The 2π phase ambiguity problem is avoided by using a light source with short coherence length. In this case, the fringes of the interferogram are narrowly localized into a path length difference region so the variation of the path length difference can be determined without the 2π ambiguity by locating the fringe peak or the envelope peak of the interferogram. This type of interferometry is known as white-light or low-coherence interferometry (LCI).
Optical systems based on low-coherence interferometry have been widely studied and have been successfully developed in commercial applications such as the optical coherence tomography (OCT) and the low-coherence profilometry (LCP) which have become standard measurement tools. One gains two-dimensional cross-section image information using OCT or surface depth information using LCP by forming an interferometer between test and reference surfaces and by scanning the path length difference between them through a range of values. These optical measurement systems are mostly aimed to laboratory type or similar applications where environmental conditions are less of a concern.
Optical sensing systems based on LCI and aimed to industrial applications and to other non-laboratory type applications have also emerged as commercial products but they are still not standard measurement tools and there are just a few number of commercial suppliers. For these types of applications it is required that the sensing part of the measurement system must be significantly separated from the signal conditioning or readout part. In this case, the optical sensor based on LCI needs two interferometers usually connected with an optical fiber: 1) the sensing interferometer, which is subjected to the physical magnitude to be measured, and 2) the readout interferometer which is used to measure the measurand-induced changes of the path length difference of the sensing interferometer. This so-called tandem interferometer arrangement is generally more complex than that of the single interferometer configuration.
The optical sensors aimed at industrial applications and other non-laboratory type applications are likely to be exposed to severe environmental conditions. It is therefore important that the sensing interferometer must be designed to be sensitive to one type of measurand and to limit the spurious effects of other mesurands. It must also yield a constant and ideally linear relationship between the path length difference and the measurand. In the same manner, the readout interferometer must be very stable, that is, its internal calibration must remain valid for a long period of time. It must also provide a constant reading with a minimum dependence on environmental factors such as temperature, vibration, etc. These industrial “must-have” requirements adding to economic constraints prevented many optical sensing technologies developed in the laboratories from reaching the industrial sensor and other non-laboratory marketplaces.
A number of optical sensors for measuring a physical magnitude have been already proposed. U.S. Pat. No. 4,140,393 Cetas, February, 1979 and U.S. Pat. No. 4,598,996 Taniuchi, July, 1986 disclose the use of different birefringent crystals in a two-beam interferometer configuration as the sensing element for measuring temperature. They use crystals such as LiTaO3, LiNbO3, BaTiO3 and SrxBa1-xNb2O6 to form a polarization sensing interferometer and they measure the light intensity at the output of the interferometer which varies sinusoidally due to temperature-induced changes of the crystal birefringence. Their optical sensing system is based on narrow-band light source so their measuring technique suffers from the 2π phase ambiguity and therefore offers a limited measurement range.
U.S. Pat. No. 5,255,068 Emo et al., October, 1993 uses similar crystals and sensing interferometer arrangement than Cetas and Tanaiuchi for measuring temperature but their optical sensing system benefits from the low coherence properties of the light source they use. However, the light source spectrum, modulated according to the temperature-dependent birefringence of the crystal, is recorded using a dispersive spectrometer which is known to have a lower optical throughput than an interferometer. Since the resulted signal is obtained in the frequency or wavelength domain rather than in the time or spatial domain, they use a Discrete Fourier Transform signal processing method which can be time consuming without mentioning the cost and complexity of using a dispersive spectrometer configuration. Moreover, the above-mentioned crystals are known to have a strong frequency-dependence of their birefringence which can severally reduce the accuracy of the Fourier transform signal-processing method.
Also known in the art, is the document by Bosselmann and Ulrich entitled “High-accuracy position-sensing with fiber-coupled white-light interferometers” published in OFS'84, Sep. 5-7, 1984 in which they describe the use of a Michelson interferometer as the readout interferometer in a LCI-based configuration. The path length difference of the Michelson interferometer is scanned by displacing one of its two mirrors and the fringes of the interferogram are recorded at the output of the interferometer using a single photodetector. The location of the fringe peak on the interferogram is determined from the measured values taken at different scanned positions of the movable mirror. Due to its movable optical parts, the mechanical stability is the weak point of this system without mentioning the problem of having to measure the position of the movable mirror with high precision.
U.S. Pat. No. 5,392,117 Belleville et al., February, 1995, U.S. Pat. No. 5,349,439 Graindorge et al., September, 1994 and the document by Duplain et al. “Absolute Fiber-Optic Linear Position and Displacement Sensor” published in OSA Technical Digest Series, Vol. 16, 1997 describe the use of a Fizeau interferometer for the measurement of the path length difference of a sensing interferometer. Their LCI-based optical sensing system consists of recording the fringes of the interferogram at the output of a Fizeau readout interferometer using a linear photodetector array and to locate the fringe peak position on the interferogram. The Fizeau interferometer, although it has no moving part, is made of an air-spaced wedge or a solid optical wedge which is not easy to produce as it requires the use of very complex thin film deposition methods or the use of optical component manufacture and assembly methods with severe thickness tolerances, optical alignment, material stability and optical quality. Moreover, the Fizeau interferometer, even tough it can be fabricated with a low finesse, still remains a multiple-beam interferometer in which case, the visibility of the fringes, when used in LCI configuration, is generally lower than that of a two-beam interferometer.
U.S. Pat. No. 4,814,604 and U.S. Pat. No. 4,867,565 issued to Lequime, as well as the document by Mariller and Lequime entitled “Fiber-Optic White-Light birefringent temperature sensor” published in SPIE Proceedings, Vol. 798, 1987, disclose the use of a LCI-based optical sensing device including a polarization sensing interferometer similar to the configuration disclosed in Cetas and Taniuchi patents. Their LCI-based optical sensing system consists of recording the fringe pattern at the output of a polarization readout interferometer using a linear photodetector array or a single photodetector. Their polarization readout interferometer is a rather complex assembly of different birefringent elements placed in between two polarizers. The birefringent elements comprise, at least, a crystal plate with two elementary birefringent prisms stuck together along a face so to form a Wollaston or a modified-Wollaston prism. These birefringent elements are mounted in variant forms of the Babinet compensator and the Soleil compensator. These types of configurations produce complex assembly devices and suffer from important drawbacks. In it simplest configuration, the plane of localization of the fringes is inside the Wollaston prism and is inclined to the exit face of the Wollaston prism. This situation requires correction optics to form an image of the fringes onto the surface of the photodetector. However, the inclination of the plane of localization produces a residual focusing error at the surface of the photodetector and therefore leads to a reduction in the fringe contrast unless the light source has a high degree of spatial coherence. To prevent this situation, Lequime proposes some modifications in their configuration by using a second Wollaston prism and an achromatic halfwave plate, but at the expense of increasing the complexity of the device.
Due to the high birefringence dispersion of the crystal used in their sensing interferometer (and possibly in the readout interferometer) the interferogram can be severally distorted therefore compromising the localization of the envelope peak or the fringe peak. They propose two solutions to overcome this problem. One solution consists to have their readout interferometer made of same birefringent material to that of the sensing interferometer. Such solution is likely to increase the sensitivity of the readout interferometer to environmental influences and therefore is not desired for industrial-based applications. Another solution proposed is to use a light source with narrower spectrum where the dispersion effect can be neglected but this solution comes to the expense of widening the path length difference region of the fringes which reduce the accuracy of the envelope peak or fringe peak location.